Question:

In a biprism experiment a steady interference pattern is observed on the screen using a light of wavelength \(5000 \mathop {\rm{A}}\limits^{\rm{o}}\). Without disturbing the set up of the experiment, the source of light is replaced by a source of wavelength \(6400 \mathop {\rm{A}}\limits^{\rm{o}}\).

The fringe width will

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Fringe width is directly proportional to the wavelength when everything else is unchanged.
Updated On: Nov 13, 2026
  • decrease by \(48\%\)
  • decrease by \(28\%\)
  • increase by \(48\%\)
  • increase by \(28\%\)
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The Correct Option is D

Solution and Explanation

Step 1: Understanding the Concept
In a biprism (or double slit) setup the fringe width is \(\beta=\dfrac{\lambda D}{d}\). With the setup unchanged, \(\beta\propto\lambda\).

Step 2: Take the ratio
\(\dfrac{\beta_2}{\beta_1}=\dfrac{6400}{5000}=1.28\)

Step 3: Calculation
The fringe width becomes 1.28 times, an increase of \(0.28=28\%\). This matches option (D).

Final Answer:
The fringe width increases by 28 percent. \[ \boxed{\text{(D) }\text{increase by }28\%} \]
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